fix(tokenizer): resolve prepend_id scope issue in encode()

[heloanc]

Summary

Fix NameError and ValueError in Tokenizer.encode() when prepend is a string.

Problem

1. prepend_id is assigned inside if prepend is not None: but referenced in a nested block — if encode_single_token() raises, prepend_id is never defined
2. encode_single_token() raises ValueError for multi-token prepend strings
3. Inner guard checks prepend instead of prepend_id, masking failed computation

Fix

• Initialize prepend_id = None before conditional blocks
• Wrap encode_single_token() in try/except with fallback to encode_ordinary()
• Check prepend_id is not None instead of prepend is not None for insertion

Fixes #348